The present invention relates to a printing process for the transfer of printing substance from an ink carrier onto an imprinting material, the printing substance undergoing a change in volume and/or position by means of an induced procedure of an energy-releasing apparatus, and thereby a transfer of a printing point onto the imprinting material takes place, as well as a printing machine for this.
By a printing process is meant primarily a process for the reproduction as often as required of text and/or image patterns by means of a printing plate which is re-inked after each impression. In general, a distinction is made here between four basically different printing processes. Firstly, the relief printing process is known, in which the printing elements of the printing plate are raised, while the non-printing parts are recessed. This includes for example letterpress printing and so-called flexographic or aniline printing. Furthermore, flatbed-printing processes are known in which the printing elements and the non-printing parts of the printing plate essentially lie in one level. These include offset printing, but also processes known more in the artistic field, such as e.g. lithographic printing. In the case of offset printing, strictly speaking the inked drawing on the printing plate is not printed directly onto the imprinting material, but is first transferred onto a rubber cylinder or a rubber blanket and only then is the imprinting material printed from this. Where in the following reference is made to imprinting material, however, this is to be understood as both the actual imprinting material, i.e. the material to be printed on, and a chosen transfer means, such as e.g. a rubber cylinder. A third process is the so-called gravure printing process in which the printing elements of the printing plate are recessed. This includes a series of manual techniques, such as e.g. copper engraving and etching. A gravure printing process used industrially is rotogravure printing. Finally, a porous printing process is also known which is sometimes also called the screen-printing process, in which at the printing positions the ink is transferred onto the imprinting material through screen-like openings of the printing plate.
These printing processes are all characterized by the fact that they require a printing plate which is more or less costly to produce, with the result that these printing processes operate economically only with very long print runs, usually well over 1000 units. Thus for example when producing a relief printing plate, a screen film of the pattern to be printed must initially be created which is copied onto the material of the printing plate by means of a light-sensitive layer. As the non-printing parts of a relief printing plate must be recessed relative to the printing parts, the metal printing plates are then etched or plastic printing plates are washed out. However, these printing plates can be used only for the printing of a specific pattern. If another pattern is to be printed, a new relief printing plate must be produced.
For the printing of short print runs, printers are already used which in general are connected to an electronic data processing system. In general, these use digitally triggerable printing systems which are in a position to print individual printing points as required. Such printing systems use various processes with different printing substances on different imprinting materials. Some examples of digitally triggerable printing systems are: laser printers, thermal printers and ink jet printers. Digital printing processes are characterized by the fact that they do not require printing plates.
An electro-thermal ink jet printing process is known for example from GB 2 007 162, in which the water-based ink is briefly heated in a suitable ink jet by electrical pulses until it boils, with the result that a gas bubble suddenly develops and an ink droplet is shot out of the jet. This process is generally known by the term “bubblejet”. However, these thermal ink printing processes have the disadvantage that on the one hand they consume a great deal of energy for the printing of an individual printing point and on the other hand they are suitable only for printing inks which are water-based. Furthermore, every single printing point must be triggered separately by the jet. In contrast, piezoelectric ink printing processes suffer from the disadvantage that the required jets are easily blocked, with the result that only special and expensive inks can be used for this.
In addition, it is known from DE 195 44 099 that with the help of a laser beam or electro-thermal heating means, solid printing substances can be melted and thereby transferred. A transparent cylinder is homogeneously provided with small cells on the surface. These cells are then filled with molten liquid ink and wiped using customary processes. The ink to be printed is then melted in a targeted manner from the inside through the cylinder by firing a laser beam or by electro-thermal processes and is thus drained and thereby a printing point is set. Also in this process, the choice of printing substance is very limited because it is necessary for the printing process that the printing substance shows a phase transition from the solid into the liquid phase as quickly as possible and in an energy-saving manner. Furthermore, in this printing process the filling of the cells with meltable ink is problematic.
Finally, it is known from DE 197 46 174 that a laser beam, through very short pulses in a printing substance which is located in cells of a printing roller, induces a procedure with the result that the printing substance undergoes a change in volume and/or position. The printing substance thereby spreads over the surface of the printing plate and it is possible to transfer a printing point onto an imprinting material moved up against same. However, in this process it is disadvantageous that the filling of the cells is very difficult due to the small diameter of the cells.